In the fabrication of ceramic substrates or ceramic chip carriers, conductive metal pattern for wiring and via metallurgy on substrate is defined by screening metal-polymer composite paste on green sheet by screening through a contact mask, such as a metal mask with closely spaced fine dimension etched pattern. Paste screening through such masks leaves paste residue on the mask surface as well as on the side walls of the mask features. This residue must be removed after one or more screening passes in order to assure consistent quality defect-free screened pattern.
Standard process for ceramic substrate production involves green sheet casting of ceramic slurry on a carrier, green sheet blanking, via punch, circuit personalization by screening of conductive paste through a mask screen, followed by singulation/sizing, inspect/collate/register, stack and lamination, binder bum-off and sintering operation.
The ceramic slurry typically comprises ceramic filler, polymer binder, solvent vehicle and dispersing agent.
The conductive pastes commonly used in multilayer ceramic (MLC) technology are dispersions of metal powder, polymer binder, and a high boiling solvent system along with suitable surfactants, dispersants, and thickening agents to obtain desired screening characteristics.
Commonly used metal fillers are molybdenum, copper, tungsten, and nickel in conjunction with polymer binders of the type, ethyl cellulose, acrylate polymers as poly(methyl methacrylate), poly(ethyl methacrylate) and related systems, and polyhydrocarbons based thermoplastic resins, which are all hydrophobic polymer systems.
Representative high boiling solvents used in metal-polymer dispersions are ester-alcohol such as 2,2,4-trimethylpentane diol-1,3-monoisobutyrate (Texanol), glycol ether type solvents, such as, di(ethyleneglycol) monomethyl ether, di(ethyleneglycol) monobutyl ether acetate or butyl carbitol acetate (BCA), di(propyleneglycol) monomethyl ether, di(propylene glycol) monobutyl ether and the like.
Solids component of the pastes used typically comprise between about 70 to about 85% by weight of metal filler, between about 1 to about 5% of organic binder and other necessary organic additives, and the balance constituting the solvent vehicle system. Depending on the binder-solvent system used in paste formulations, the paste can be polar or non-polar, hydrophilic or oleophilic and thus having different wetting characteristics for water-based cleaning and also to having different interface interactions and adhesion to various metal mask surfaces. Selection of a particular paste type in terms of the metal filler and its particle size distribution, the polymer binder and the solvent system is dictated by the requirement of the desired circuit pattern, conductivity of metallurgical pattern, and compatibility of the paste with the ceramic green sheet material.
In high throughput screening in the production of ceramic substrates, mask cleaning has been carried out with organic solvents in a real time operation to provide cycle time compatibility with manufacturing screening and cleaning requirements, such as described in U.S. Pat. No. 4,362,486 (Davis) and U.S. Pat. No. 4,483,040 (Magee), assigned to International Business Machines Corporation, Armonk, USA, and the disclosures of which are incorporated herein by reference.
With organic solvent-based cleaning, there are issues of flammability or combustibility, chemical safety, and waste disposal and associated cost, and therefore, aqueous cleaning is generally preferred for high volume production environment.
U.S. Pat. No. 5,221,362 (Porta) describes non-halogenated aqueous degreasing compositions based on mixture of alkanoic acid including hydroxy alkanoic acid, straight chain alkanol or an alkyl ester and water for use in removing inks and cleaning printed circuit boards.
U.S. Pat. No. 5,264,047 (Winston) describes aqueous cleaning compositions comprising alkaline salts, organic adjuvants, and a low foam hydrotrope comprised of alkali metal salt of carboxylic acids having a chain length of 7-13 carbon atoms. These compositions are used in removing soldering flux and other residue from printed wiring boards.
U.S. Pat. No. 5,279,756 (Savio) discloses non-phosphate machine dishwashing detergents, such as, alkali metal carbonate, bicarbonate, and hydroxy carboxylic acid as complexing agent.
U.S. Pat. No. 5,286,403 (O'Dwyer) describes phase-stable, aqueous cleaning concentrates containing an inorganic acid and organic acid with surfactants for cleaning floor tiles.
U.S. Pat. No. 5,464,553 (Winston) is concerned with aqueous cleaning compositions comprising alkali metal salt as carbonate/bicarbonate, organic adjuvants, and a hydrotrope for solder flux removal.
U.S. Pat. No. 5,575,857 (Lunski) is concerned with aqueous alkaline cleaning concentrate for removing inorganic salt or scale from metal surface, the cleaning solution consisting of sodium and potassium carbonate salt and a hydrotrope as sodium, potassium, ammonium, and alkanolammonium salt of sarcosinate.
U.S. Pat. No. 5,593,504 (Cala) describes method of cleaning solder paste from substrates with an aqueous cleaner, such as, alkali metal carbonate, a surfactant, alkali metal silicate, and a stabilizer for silicate, to provide a safer replacement for alcohol solvents.
As stated earlier, this invention is concerned with cleaning masks and associated equipment used in screening conductive paste onto green sheet in the manufacture of multilayer ceramic substrates. The invention is also applicable to cleaning metal stencils used in the deposition of solder paste pattern on substrates. This invention is especially concerned with aqueous cleaning of screening masks with water-based cleaning compositions that are, preferably biodegradable and/or are compatible with standard wastewater treatment methods.
According to this invention, it has been found that alkaline solutions comprising .alpha.-hydroxy acid salts, particularly, sodium, potassium, and tetraalkyl ammonium salt of lactic acid, aspartic acid, citric acid, gluconic acid, glycolic acid, tartaric acid, and related systems in the presence of excess alkali and optionally a surface active agent provide highly effective cleaning solutions for removing screening paste residue from metal masks, such as, Cu, Cu/Ni, Cu/Ni/Cu, Mo, stainless steel, to name a few. These metal masks can be electroform masks, stencils, etc.
It has been found that aqueous solution of lactic acid, aspartic acid and/or gluconic acid, sodium and/or potassium salt, or tetramethylammonium salt with pH adjusted to between 11.5-12.8 by adding excess alkali, and optionally a low foam non-ionic and/or an amphoteric surfactant or an ionic surfactant provides excellent cleaning system for screening masks to remove Mo, Cu, Cu/Ni, and tungsten based polar and non-polar pastes using pressure spray or ultrasonic agitation. In high pressure cleaning of screening masks in the MLC production using 130-180 psi spray pressure and 140-170.degree. F. solution temperature, cycle time for mask cleaning can be less than 20 seconds, on an automated screening and cleaning tool. These compositions are also found effective in cleaning solder paste residue from the metal stencils and screens. Such pastes generally consists of solder filler, such as, Pb/Sn eutectic, Pd/Sn/Pd, Ag/Pd, Pb/In, Sn/Bi, Sn/In, in organic carrier/solvent and fluxing agent, such as, rosin-based flux, organic acids, to name a few.